All the vertebrates are included in a single phylum—Chordata. This term has supplanted in zoölogy the old term Vertebrata (now reserved as a class name only), because it is more comprehensive and precise. Professor Harmer says:

"The axis of the backbone of all vertebrates is formed by an elastic rod known as the 'notochord,' which lasts throughout life in some of the lowest forms, but in the higher forms appears only in the embryo. The universal occurrence of this structure has been regarded as the most important characteristic of the Vertebrata and their allies, which are accordingly grouped together in the phylum Chordata. The members of this phylum are further distinguished from other animals by several important features. Of these one of the most important appears to be the existence of lateral outgrowths of the pharynx, which unite with the skin of the neck and form a series of perforations leading to the exterior. These structures are the gill slits, and in the fishes their walls give rise to vascular folds or gills. With the assumption of a terrestrial life the higher vertebrates lost their gills as functional organs, respiration being then performed by entirely different organs, the lungs. But even in these cases, the gill slits appear in the embryo.... Another fundamental characteristic of the Chordata is given by the central nervous system, which lies entirely above the alimentary canal, just dorsal to the notochord. Not only does this position of the nerve centers distinguish the Chordata from the Invertebrates, but a further point of difference is found in the development."

This definition requires the inclusion of various creatures very unlike "vertebrates," and the phylum therefore embraces three subdivisions: 1. Adelochorda—marine wormlike creatures having a notochord in the anterior of the body, and gill slits, both persistent; 2. Urochorda—the ascidians or tunicates, small marine creatures, some fixed along shores, others free-swimming and in some cases united into swimming colonies (e. g., the salpæ), the tadpolelike larvæ of which show a notochord in the tail; and 3. Vertebrata.

This last great subphylum is divisible into seven grand natural groups with the rank of classes, namely:

1. Acrania —Lancelets (Amphioxus).

2. Cyclostomata —Lampreys; hags.

3. Pisces —Fishes.

4. Amphibia —Amphibians.

5. Reptilia —Reptiles.

6. Aves —Birds.

7. Mammalia —Mammals.

The first of these seven classes, the Acrania, has usually, heretofore, been set apart as a subdivision equal in rank to the subphyla Adelochorda and Urochorda, and the remaining six classes were grouped into a coordinate subphylum Craniata, denoting that they alone have a distinct head (cranium); the reason was that its members, the lancelets, have no spine, but only a notochord, which, however, extends from end to end of the body above the digestive organs, and persists in the adult and throughout life. The lancelets (amphioxus) are small, fish-shaped creatures that burrow in the sand of the seashore, usually leaving only the head exposed, and sucking in a continuous current of water which brings with it minute food. They breathe through gill slits. The reproduction is bisexual, and by eggs.

The significance of the Acrania in this phylum is that they represent a very early ancestral stage of the stock from which the higher vertebrates (Craniata) have developed, and from which they themselves, of course, have also diverged to a certain degree; and it is because they retain many primitive characteristics that the study of their life histories has engaged the attention of so many eminent zoölogists and has thrown so much light on the evolutionary history of the "higher animals," or vertebrates.